Chemical Looping Combustion method or CLC: in the text hereafter, what is referred to as CLC (Chemical Looping Combustion) is an oxidation-reduction or redox looping method on an active mass. It can be noted that, in general, the terms oxidation and reduction are used in connection with the respectively oxidized or reduced state of the active mass. The oxidation reactor is the reactor where the redox mass is oxidized and the reduction reactor is the reactor where the redox mass is reduced.
In a context of increasing world energy demand, capture of carbon dioxide for sequestration thereof has become an imperative necessity in order to limit greenhouse gas emissions harmful to the environment. The Chemical Looping Combustion (CLC) method allows to produce energy from hydrocarbon-containing fuels while facilitating capture of the carbon dioxide emitted during the combustion.
The CLC method consists in using redox reactions of an active mass, typically a metal oxide, for splitting the combustion reaction into two successive reactions. A first oxidation reaction of the active mass, with air or a gas acting as the oxidizer, allows the active mass to be oxidized.
A second reduction reaction of the active mass thus oxidized, by means of a reducing gas, then allows to obtain a reusable active mass and a gas mixture essentially comprising carbon dioxide and water, or even syngas containing hydrogen and carbon monoxide. This technique thus enables to isolate the carbon dioxide or the syngas in a gas mixture practically free of oxygen and nitrogen.
The combustion being globally exothermic, it is possible to produce energy from this method, in form of vapour or electricity, by arranging exchange surfaces in the active mass circulation loops or on the gaseous effluents downstream from the combustion or oxidation reactions.
U.S. Pat. No. 5,447,024 describes a chemical looping combustion method comprising a first reactor for reduction of an active mass by means of a reducing gas and a second oxidation reactor allowing to restore the active mass in its oxidized state through an oxidation reaction with wet air. The circulating fluidized bed technology is used to enable continuous change of the active mass from the oxidized state to the reduced state and from the reduced state to the oxidized state thereof.
The active mass going alternately from the oxidized form to the reduced form thereof and conversely follows a redox cycle.
Thus, in the reduction reactor, active mass (MxOy) is first reduced to the state MxOy-2n-m/2 by means of a hydrocarbon CnH that is correlatively oxidized to CO2 and H2O, according to reaction (1), or optionally to a mixture CO+H2, depending on the proportions used.CnHm+MxOy→nCO2+m/2H2O+MxOy-2n-m/2  (1)
In the oxidation reactor, the active mass is restored to its oxidized state (MxOy) on contact with air according to reaction (2), prior to returning to the first reactor.MxOy-2n-m/2+(n+m/4)O2→MxOy  (2)
In Equations (1) and (2) above, M represents a metal.
The efficiency of the circulating fluidized bed chemical looping combustion (CLC) method is based to a large extent on the physico-chemical properties of the redox active mass.
The reactivity of the redox pair(s) involved and the associated oxygen transfer capacity are parameters that influence the dimensioning of the reactors and the rates of circulation of the particles.
The life of the particles depends on the mechanical strength of the particles and on the chemical stability thereof.
In order to obtain particles usable for this method, the particles involved generally consist of a redox pair or a series of redox pairs selected from among CuO/Cu, Cu2O/Cu, NiO/Ni, Fe2O3/Fe3O4, FeO/Fe, Fe3O4/FeO, MnO2/Mn2O3, Mn2O3/Mn3O4, Mn3O4/MnO, MnO/Mn, Co3O4/CoO, CoO/Co, and of a binder providing the required physico-chemical stability. Interactions with supports such as Al2O3 are possible.
Liquid feeds, in particular those referred to as “heavy” liquid feeds, i.e. with a high carbon to hydrogen ratio, produce a large amount of greenhouse gas. Thus, the combustion of these fuels is a particularly interesting application for CLC.
Although many studies with gas feeds (essentially methane) and solid feeds have been carried out and have shown the feasibility of the chemical combustion loop for this type of fuels, few satisfactory solutions have been provided for the combustion of heavy liquid feeds.
The principle of fluidized-bed redox chemical looping combustion of liquid feeds is known and it is for example described in patent FR-2,930,771. In relation to CLC methods applied to gas or solid feeds, chemical looping combustion of liquid feeds involves the specific features described below.
The liquid hydrocarbon feed is injected into the dense bed of the reduction reactor and it is preferably atomized within the fluidized bed so as to form fine droplets. Part of the liquid feed is vaporized on contact with the hot redox active mass, in a medium generally above 700° C., and the other part condenses so as to form a coke deposit on the surface of the redox mass due to the thermal cracking resulting from the liquid fuel exposure to very high temperatures. The heavier the feeds, the more they tend to form large amounts of coke. Thus, on a vacuum gas oil or distillate, the amount of coke formed is of the order of 1 to 20% of the feed injected. On an atmospheric residue or a vacuum residue, the amount of coke formed ranges from 10 to 80% depending on the nature of the feed injected. This coke formation depends on the nature of the feeds (coke precursor concentration, determinable by measuring the asphaltene content or the Conradson carbon content). It also depends on the contacting conditions (temperature, ratio of the hydrocarbon flow rate to the active mass flow rate, diameter of the liquid feed droplets, particle diameter, etc.) that govern the heat transfer between the particles and the droplets, and therefore the competition between the physical vaporization phenomenon and the chemical degradation linked with the thermal cracking of the hydrocarbons. After contacting the liquid feed with the redox active mass, two types of combustion reactions occur between the hydrocarbons and the redox active mass. A first reaction consists of the oxidation of the vaporized liquid fuel on contact with the redox active mass. A second reaction, slower than the first, corresponds to the gasification of the coke deposited on the particles producing syngas (CO+H2), which will then rapidly burn with the redox active mass.
This coke formation on the redox mass is a major problem in the field of CLC of liquid hydrocarbon feeds, notably because it causes a significant decrease in the oxygen transfer capacity of the redox active mass. As a consequence, the performances of the CLC process are generally degraded, whether in terms of CO2 capture, of energy production or of syngas production, depending on the applications considered for the CLC process.
Furthermore, insofar as coke formation contributes to slowing down the combustion reactions, it is in general advisable to minimize the formation of coke upon liquid feed injection so as to have the fastest possible combustion reactions.
Finally, the formation of coke on the redox mass is a constraint on the residence time of the active mass in the combustion reactor, which needs to be long enough for all the coke of the particles returning to the combustion zone, after passing through the oxidation reactor, to be removed through the slow gasification process.
Patent FR-2,936,301 provides, in order to overcome the various problems related to the formation of coke upon combustion of liquid feeds, an arrangement of reaction zones suited to the chemical looping combustion of liquid feeds, allowing controlled and optimum injection of liquid on contact with the metal oxides, and control of the particle residence time in the combustion zone. Thus, combustion of the coke deposited on the particles is predominantly achieved in the combustion reactor, which allows to limit the amount of CO2 released in the gaseous effluents at the oxidation reactor outlet and to reach a very high CO2 capture ratio (defined as the ratio of the CO2 discharged in the fumes of the combustion zone through metal oxide reduction to the total CO2 emissions of the process). However, the amounts of oxide involved have to be high in order to enable a long residence time of the coke deposited on the particles and gasification thereof.